The present invention relates to digital loop carriers, and more particularly to a digital loop carrier of a digital loop carrier system disposed between switching unit and a subscriber.
Because of the scale and cost of an exchange, there is a limit to the number of exchanges installable. Hence, there is a need to dispose a digital loop carrier between the switching unit and the subscriber so that a part of the functions of the switching unit can be executed by the digital loop carrier.
As shown in FIGS. 1 and 2, when a conventional digital loop carrier 21 is disposed at a remote location with respect to a switching unit 22, the connection between the digital loop carrier 21 and the switching unit 22 is implemented by a primary group signal (sometimes referred to as a T1 digital signal) transmission line 23 using a copper wire.
Specifically, FIG. 1 shows an IDLC (integrated digital loop carrier) configuration in which 24 digital voice channels may be multiplexed together over a 4-wire cable between the digital loop carrier 21 and the switching unit 22. The digital loop carrier 21 demultiplexes the signals on the channels and performs digital-to-analog conversion before delivering the signals to a subscriber 24.
FIG. 2 shows a UDLC (universal digital loop carrier) configuration in which a digital loop carrier 25 is disposed in the exchange 20, and the digital loop carrier 25 is connected with the switching unit 22 via a subscriber interface 26. Signals arriving at the digital loop carrier 25 via the subscriber interface 26 are subjected to analog-to-digital conversion and the multiplexed. The multiplexed signals are transmitted to the other digital loop carrier 21 disposed at a remote location via the primary group signal transmission line 23. The digital loop carrier 21 demultiplexes the arriving signals and subjects them to digital-to-analog conversion. The analog signals are then transmitted to the subscriber 24.
FIG. 3 shows the internal construction of the digital loop carrier 21 (DLC-RT) disposed at a remote location with respect to the exchange.
A description will first be given of the downstream signal. The T1 signal (bipolar signal) from the switching unit 22 is received at a line termination part 27 via the transmission line 23 and changed to a unipolar signal.
The unipolar signal is sent to a low-speed multiplexing/demultiplexing part (DSI.MUX/DMUX part) 28 for synchronizing detection so as to obtain a DS0 signal (64k b/s) corresponding to one telephone line. In case the subscriber 24 subscribes to a normal telephone service, a subscriber interface 29 performs digital-to-analog conversion and sends the analog signal to the subscriber 24. A signaling bit superimposed on a main signal sent from the switching unit causes a bell of the subscriber 24 to ring.
A description will be given of the upstream signal. The subscriber interface 29 performs off-hook supervision of the subscriber 24, notifying the exchange unit of an occurrence of a call by means of a signaling bit superimposed on a main signal. The subscriber interface 29 also performs analog-to-digital conversion. The low-speed multiplexing/demultiplexing part 28 multiplexes 24 channels of DS0 signals (64k b/s) from the subscriber interface 29 so as to produce a frame of a DS1 signal (1.5M b/s) The line termination part 27 converts the unipolar DS1 signal to a bipolar signal and sends the same to the exchange. For protection of the transmission between the exchange and the remote station, there may be employed a redundant configuration in which one reserve line termination part is provided for each set of four line termination parts 27 so that a ratio of 4:1 with respect to the transmission line 23 results.
The above described digital loop carrier 21 has a function of concentrating 48 subscriber channels into 24 channels for efficient usage of the T1 signal transmission line 23.
It is expected that as the multimedia technology is used more extensively in the ordinary household, there will be a need to provide, in addition to the conventional service mainly designed for telephone, a digital signal service such as the ISDN, an image service such as the one dealing with video images, and a broadband transmission service using a DS1 signal, a DS3 signal or an OC-3 signal. However, it is impossible for the above described conventional digital loop carrier to transmit a large capacity of digital signals because the transmission line between the digital loop carrier and the switching unit is a T1 signal transmission line using a copper wire.
Another disadvantage of the conventional digital loop carrier is that, because only the copper wire is used for connection, optical fibers can be installed between the exchange and the remote station only after providing optical transmission terminal unit.
Still another disadvantage of the conventional digital loop carrier is that connections of cables must be manually done when the cables are installed newly and when a modification is made to the connection, cables carrying the DS1 signal and the DS0 signal being connected by using a connector panel.
Still another disadvantage of the conventional digital loop carrier is that the conventional concentration function of concentrating 48 channels into 24 channels lacks flexibility. In order to improve a circuit usage efficiency and a block ratio, there is a need to increase channels subjected to the concentration.
Still another disadvantage of the conventional digital loop carrier is that, since the subscriber line is installed outside, it can be easily affected by external noise or shock. This susceptibility to failure requires that a leader line terminal to which measuring instrument can be connected be provided so that the maintenance of the digital loop carrier can be performed.